The 1982 worldwide market for photovoltaic power generating modules was served almost exclusively by polycrystalline silicon and single crystalline silicon. The best modules made from these materials were 10% efficient and cost $6./peak watt when purchased in large quantity. To achieve significant penetration of the U.S. central station power generation market, flat plate modules must be manufactured for $25./m.sup.2 to $50./m.sup.2 with efficiencies of 15% to 25%. The high cost associated with polycrystalline silicon and single crystal silicon is a consequence of the indirect absorption of light. This necessitates a photovoltaic cell approximately 300 microns thick.
Alternative materials which have been investigated for photovoltaic use are thin film amorphous silicon alloy photovoltaic devices and heterojunction photovoltaic devices based on CdS and (Cd,Zn)S alloys. These materials are amenable to continuous mass production and cost projections for modules with 10% efficiency or greater are less than $1./peak watt.
Amorphous semiconducting alloys have been considered for use in photovoltaic devices to convert sunlight into electricity. One alloy which has received scrutiny is hydrogen containing amorphous silicon. This alloy can be deposited over large areas from the gas phase by glow discharge decomposition of silicon bearing gases. The thickness of a photovoltaic cell composed of amorphous silicon need only be 1/2 micron thick for the efficient collection of sunlight. Hydrogen containing amorphous silicon (hereinafter called amorphous silicon) refers to the material formed by glow discharge decomposition of silane as described by Carlson in U.S. Pat. No. 4,064,521, the details of which are incorporated herein by reference thereto. The amorphous silicon may also contain other elements such as fluorine. An example is the glow discharge decomposition of SiF.sub.4 +H.sub.2 mixtures as taught in U.S. Pat. No. 4,226,898, the details of which are incorporated herein by reference thereto.
Photovoltaic devices (hereinafter called photovoltaic cells) of amorphous silicon have achieved AMl efficiencies of 10% for devices 1 cm.sup.2 in area. Three photovoltaic heterojunction cells utilizing CdS have reached laboratory efficiencies of 10%. These are CdS-CdTe and CdS-CuInSe.sub.2 and CdS-Cu.sub.2 S. All three heterojunction structures are considered "Thin Film" solar cells because they can be fabricated with a thickness of 20 microns or less. CdS-Cu.sub.2 S refers to the method and material as described by Carlson et al, U.S. Pat. No. 2,820,841, the details of which are incorporated herein by reference thereto. CdS-CuInSe.sub.2 refers to the method and material as disclosed in Mickelsen U.S. Pat. No. 4,335,266.
The efficiency of the amorphous silicon photovoltaic cells and the CdS based photovoltaic heterojunction cells can be increased by 50% if combined into a photovoltaic tandem cell.
Dalal discloses in U.S. Pat. No. 4,387,265 a photovoltaic tandem cell comprising at least two p(+) i n(+) amorphous cells in optical series and sharing a transparent ohmic contact layer.
Dalal also discloses a photovoltaic CdS-Cu.sub.2 S, CdS-Cu.sub.2 S tandem cell.
Hanak, in U.S. Pat. No. 4,292,092, discloses a method for laser scribing a plurality of semiconductor layers including amorphous silicon, CdS, and Cu.sub.2 S.
Hovel, in U.S. Pat. No. 4,289,920, discloses a photovoltaic amorphous cell in optical series with a photovoltaic crystalline cell, said cells separated by an insulator.
Hovel, in U.S. Pat. No. 4,292,461, discloses a tandem cell having a higher bandgap amorphous cell and a lower bandgap photovoltaic homojunction crystalline cell joined by an optically transparent conducting layer.
Dalal, in U.S. Pat. No. 4,253,882, discloses a photovoltaic amorphous silicon-crystalline silicon homojunction tandem cell.
A detailed calculation by the inventor has revealed that an amorphous silicon photovoltaic cell in conjunction with a CdS-Cu.sub.2 S heterojunction photovoltaic cell is an ideal combination of materials for a photovoltaic tandem cell. Amorphous silicon is the only material which can easily solve the problem of current matching in the photovoltaic solar cell. This is possible because of the unique mode of decomposition of amorphous silicon, glow discharge decomposition. The processing sequence and processing temperatures for the fabrication of the proposed photovoltaic tandem cell are compatible with glass as a substrate and the amorphous silicon being deposited thereon first.
Using current state of the art material properties for both amorphous silicon, CdS, and Cu.sub.2 S, the calculated efficiency of the proposed photovoltaic tandem cell is 15%. Large area modules with efficiencies of 10% or greater at low cost now become a realistic manufacturing possibility.
The most critical factor in determining the efficiency of a photovoltaic tandem cell (hereinafter called tandem cell) is the need to match the short circuit currents of both the top and bottom cells. The total current from a tandem cell can be no greater than the smallest current generated by either the top or bottom cell. The thickness of the amorphous silicon top cell can be precisely controlled by interrupting the glow discharge at the appropriate time. In addition, the band gap can be varied by either varying the substrate temperature during deposition or incorporating carbon, germanium, nitrogen, oxygen, or fluorine into the amorphous matrix. The use of either technique allows one to control the amount of light absorbed or alternatively, the current generated in the amorphous cell, said current being made equal to that generated in the CdS-Cu.sub.2 S cell. The efficiency of a tandem cell can be calculated by the following procedure.